FIG. 1 depicts a schematic diagram of a telecommunications system in the prior art. Telecommunications system 100 comprises:
i. telecommunications network 101,
ii. private branch exchange 102,
iii. on-premises telecommunications terminals 103 and 104,
iv. data-processing system 108,
v. off-premises telecommunications terminal 109, and
vi. network telecommunications terminals 111 and 112,
all of which are interconnected as shown.
Telecommunications network 101 comprises the Public Switched Telephone Network, which is a complex of telecommunications equipment that is owned and operated by different entities throughout the World. In the United States of America, for example, the Public Switched Telephone Network (or “PSTN”) comprises an address space that is defined by ten digits, and, therefore, comprises 10 billion unique addresses or “telephone numbers.” The public switched telephone networks in other countries are similar.
Network 101 interconnects other telecommunications networks, which include: (i) the cellular network supported by data-processing system 108 and (ii) the enterprise network supported by private branch exchange 102. The cellular network supported by system 108 provides telecommunications service to one or more cellular telecommunications terminals, including terminal 109. The enterprise network supported by exchange 102 provides telecommunications service to one or more telecommunications terminals, for example terminals 103 and 104, within the enterprise area served, such as an office building or campus.
Additionally, network 101 provides telecommunications service to telecommunications terminals that are in the Public Switched Telephone Network, such as terminals 111 and 112. The telecommunications terminals that network 101 serves might be wireline terminals or wireless terminals, or a combination of both.
Data-processing system 108 is a mobile switching center that provides service for cellular phones (e.g., terminal 109, etc.) that are registered with system 108. Alternatively, data-processing system 108 might be a different type of data-processing system than a mobile switching center, such as a wireline switch, a packet router, and so forth. Similarly, terminal 109 might be a different type of terminal than a cellular phone.
Private branch exchange (PBX) 102 is capable of switching incoming calls (e.g., from terminal 111, etc.) from the Public Switched Telephone Network of network 101 via communications paths in trunk group 120 (e.g., path 120-1, path 120-2, etc.) to one or more on-premises terminals, such as on-premises terminals 103 and 104. Private branch exchange 102 is also capable of handling outgoing calls from one or more on-premises terminals to the Public Switched Telephone Network via the communications paths that make up trunk group 120.
Private branch exchange 102 is also capable of forwarding an incoming call, such as from terminal 111, to an “off-premises” terminal, such as terminal 109, that is affiliated with exchange 102 and accessible through network 101. This type of forwarding to a terminal affiliated with exchange 102 is also known as “extending” a call because the connection to the off-premises terminal appears to exchange 102 as an additional PBX line. Exchange 102 extends the call to the off-premises terminal in addition to switching the same incoming call to an “on-premises terminal” within the enterprise area that exchange 102 serves. Terminals 103 and 104 are considered to be on-premises terminals with respect to private branch exchange 102, while terminal 109 is considered to be an off-premises terminal with respect to exchange 102. Note that in telecommunications system 100, terminals 111 and 112 are not considered to be off-premises terminals because, unlike terminal 109, they are not affiliated with exchange 102.
To accomplish (i) the switching of an incoming, enterprise-related call to an on-premises terminal and (ii) the extending of the call to the correct off-premises terminal, private branch exchange 102 maintains a table that correlates the off-premises telephone number to the on-premises, private branch exchange extension. Table 1 depicts a table that illustrates the correlation.
TABLE 1PBX Extension-to-PSTN Number DatabaseOn-PremisesTelecommunicationsPrivate BranchTelecommunicationsTerminalExchange ExtensionNetwork Number103732-555-0102, x11201-555-1236104732-555-0102, x12908-555-3381. . .. . .. . .
As an example, a first caller at terminal 111 who wishes to reach the PBX user of terminal 103 dials the PBX number (i.e., 732-555-0102). Private branch exchange 102 receives the incoming call and the extension number (i.e., x11) as specified by the caller. By using stored information that is similar to the information in Table 1, private branch exchange 102 determines that the call is also to be extended to off-premises telephone number 201-555-1236. This is the telephone number that is associated with off-premises terminal 109, which belongs to the PBX user of terminal 103. Exchange 102 then extends the call to terminal 109 via an available resource, communications path 120-1. The idea behind transmitting the call to both terminal 103 and 109 (or sometimes to terminal 109 only) is that if the PBX user is not reachable at his office phone (i.e., terminal 103), then possibly he is reachable at a phone that is outside of the office (i.e., terminal 109). The extending of the call to an off-premises terminal enhances the caller's experience by only requiring the caller to use a single telephone number to reach the PBX user, regardless of whether or not the PBX user is in the office.
Continuing with the example, a second caller at terminal 112, who also wishes to reach the PBX user of terminal 103, dials the same PBX number as the first caller (i.e., 732-555-0102). Private branch exchange 102 receives the second incoming call and the extension number (i.e., x11) as specified by the second caller, and determines that the second call is also to be extended to off-premises telephone number 201-555-1236, associated with terminal 109. Exchange 102 then extends the second call to terminal 109 via the next available resource, communications path 120-2.
At this point in the example, data-processing system 108 is handling two calls for terminal 109 on two separate communications paths from private branch exchange 102. If terminal 109 terminates only a single PBX line, also known as a “call appearance,” data-processing system 108 is responsible for holding the waiting, second call until the user of terminal 109 signals (i.e., “flashes”) data-processing system 108 to: (i) put the first call on hold, and (ii) connect the second call to the terminal.